The present invention relates to a planetary gear train for an automatic transmission.
A representative example of a conventional planetary gear train for a four speed automatic transmission incorporating three planetary gear sets is one for an automatic transmission of AT540 type manufactured by General Motors in the U.S., which is shown in FIG. 1. The planetary gear train of for this automatic transmission comprises three planetary gear sets G1, G2 and G3 wherein sun gears S1, S2 and S3, ring gears R1, R2 and R3, and pinion carriers PC1, PC2 and PC3 are connected to an input shaft I, an output shaft O, two clutches C1 and C2, and three brakes B1, B2 and B3 as shown in FIG. 1. This conventional planetary gear train provides four forward speed ratios and one reverse speed ratio which have gear ratios as shown in FIG. 1A by actuating a selected combination of the clutches C1 and C2 and brakes B1, B2 and B3 as shown in FIG. 1A.
This planetary gear train, however, poses a problem as follows: That is, the gear ratio for the reverse speed is too large as compared to the gear ratio for the first forward speed. If, for example, a ratio (.alpha.1) of the number of teeth of the sun gear to the number of teeth of the ring gear in the planetary gear set G1, a ratio (.alpha.2) of the number of the teeth of the sun gear to the number of teeth of the ring gear in the planetary gear set G2 and a ratio (.alpha.3) of the number of teeth of the sun gear to the number of teeth of the ring gear in the planetary gear set G3 are equal to 0.42, the gear ratio for the first forward speed results in 3.381 and the gear ratio for the reverse results in -4.669. Thus, it is necessary to increase the strength of each of the rotary power transmitting elements such as gears, shafts and the like to withstand a large torque during the reverse drive. This means that large-sized torque transmitting component parts have to be used, resulting in an increase in size of a transmission. Increasing the strength of the overall transmission to withstand a large torque produced during the reverse drive which is less frequently used as compared to the forward speeds is not a good measure in terms of cost.